The present invention relates to an internal combustion engine, which can be operated on auto-ignition combustion of fuel with low cetane number like gasoline, and more particularly to a system and method for auto-ignition combustion management in an internal combustion engine.
When certain conditions are met within a charge of lean air/fuel mixture during low load operation, auto-ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high thermal efficiency. NOx emission produced in controlled auto-ignition combustion is extremely low. In controlled auto-ignition combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values resulting in very low NOx emission.
Auto-ignition combustion at low speeds with low or middle load and spark-ignition combustion at high speeds with high or full load result in enhanced fuel consumption rate and reduced NOx emission at low speeds with low or middle load and high power output at high speeds with high or full load.
JP-A 10-196424 discloses an auto-ignition internal combustion engine. According to this known engine, each cylinder has a control piston in addition to a reciprocating piston. Time of auto-ignition of air/fuel mixture is governed by the control piston, which provides additional compression around top dead center of reciprocating piston. With this additional compression, the air/fuel mixture can be elevated in temperature high enough for auto-ignition.
JP-A 11-210539 discloses a spark assist type auto-ignition internal combustion engine. According to this known engine, an open time of intake valve is adjusted to vary amount of burnt gas to be drawn during induction stroke of piston, thereby to vary temperature of gas within a combustion chamber. A temperature sensor is provided to detect the temperature of gas. If it is determined that the temperature of gas within the combustion chamber fail to match a target temperature level, the open time of the intake valve is adjusted to maintain the gas temperature as high as the target temperature level At the target temperature level, the air/fuel mixture starts auto-ignition if spark is produced by a spark plug.
JP-A 7-4287 discloses a control system for a diesel engine. According to this control system, oxygen density of intake charge drops due to exhaust gas recirculation (EGR) to lower burning temperature. Injection timing of fuel is adjusted to a crank position after top dead center of compression stroke to provide increased ignition delay when oxygen density of intake charge drops. This control is effective in auto-ignition combustion of fuel with high cetane number, with low emission of NOx and smoke.
Prior art techniques according to JP-A 10-196424 and JP-A 11-210539 are satisfactory to some extent in controlling beginning of burning. However, there is a limit to increasing load request because burning velocity increases rapidly to bring about a rapid increase in cylinder pressure during operation with high load, resulting in unacceptably high level of vibration and noise. Besides, there is a limit to increasing engine speed because combustion is not completed within one cycle at high engine speeds, resulting in high emission of unburned gas.
The control strategy according to JP-A 7-4287 has proven to be effective in holding conditions for stable auto-ignition combustion of fuel with high cetane number. However, applying this control strategy to a gasoline internal combustion engine does not provide satisfactory auto-ignition combustion of gasoline fuel. This is because conditions for auto-ignition combustion of gasoline fuel are considerably narrower than those for auto-ignition combustion of diesel fuel.
An object of the present invention is to accomplish stable auto-ignition combustion of gasoline fuel in an internal combustion engine over extended operating conditions with low fuel consumption and good drivability.
In one aspect of the present invention, the above object is achieved by a system for auto-ignition combustion management in an internal combustion engine having at least one cylinder, and a piston reciprocating in the cylinder, the system comprising:
an actuator to vary at least one of controlled parameters governing burning timing in the cylinder; and
a controller for varying the burning timing via said actuator such that main combustion timing retards as load request on the engine becomes high during operation within an auto-ignition combustion range.